As disclosed in Japanese Utility Model Application Laid-Open No. H6-48887, a light-emitting device (array) used in a conventional optical print head has individual electrodes formed on the top surface of the device so as to correspond one-to-one to a plurality of light-emitting sections, and has a common electrode, i.e., an electrode common to all the light-emitting sections, formed on the back surface of the device. This makes it impossible to drive the device on a time-division basis within the device. Since time-division driving is impossible, it is necessary to provide as many individual electrodes as there are light-emitting sections. Thus, as the light-emitting sections are arranged with increasingly high density, the individual electrodes need to be arranged with accordingly high density. This makes it difficult to connect the light-emitting device to its driving IC.
To solve this problem, Japanese Patent Application Laid-Open No. H6-163980 proposes a light-emitting device that permits time-division driving within the device. Specifically, in this light-emitting device, the light-emitting sections formed on it are divided into m groups, and a plurality of common electrodes are provided, with each of them connected to all the light-emitting sections belonging to one group. Moreover, n individual electrodes are provided, with each of them connected to m light-emitting sections belonging to different groups. Thus, this light-emitting device has m×n light-emitting sections. With this light-emitting device, selecting one of the m common electrodes on a time-division basis makes it possible reduce the number of individual electrodes to 1/m of the number conventionally required. This makes it easy to connect the light-emitting device to its driving IC.
FIG. 21 shows an example of the circuit configuration assumed on the basis of the conventional dynamic driving method in a case where a light-emitting device ready for time-division driving as proposed in the aforementioned publication is used. In FIG. 21, each light-emitting device 100 has a plurality of light-emitting sections formed on its top surface, and those light-emitting sections are divided into two groups. Two common electrodes are each connected to all the light-emitting sections belonging to one of the so divided groups. Moreover, on a one-to-one basis, each light-emitting device 100 is connected by wire bonding to a driving IC 200 having the same number of terminals as there are individual electrodes in the light-emitting device. All light-emitting devices 100 are connected by way of two ground lines 400 to a common electrode selecting IC 300 for choosing between the two common electrodes.
On the other hand, in Japanese Patent Application Laid-Open No. 2001-113751, the applicants of the present invention proposed a versatile driving IC suitable for time-division driving. According to this publication, a driving IC incorporates a first drive section that feeds a current to individual electrodes and a second drive section that selects one of common electrodes at a time on a time-division basis. In the drive IC configured in this way, the second drive section is provided with a plurality of output terminals that are connected individually to the common terminals, and this helps distribute the current that flows through the drive IC.
However, with the configuration shown in FIG. 21, dividing the light-emitting sections of each light-emitting device into four or more groups necessitates many conductors to electrically connect its common electrodes to the common electrode selecting IC. The differences in resistance among those conductors cause differences in brightness among the light-emitting sections depending on their position. Moreover, a voltage drop or a rise in the ground potential resulting from the internal resistance present near the terminals provided in the common electrode selecting IC adversely affects the brightness of the individual light-emitting sections when they are lit. This causes the brightness of the light-emitting sections to differ according to the number of light-emitting sections lit.
On the other hand, with the configuration proposed in Japanese Patent Application Laid-Open No. 2001-113751, a light-emitting device and the corresponding driving IC are connected together at one side thereof This makes it impossible to reduce the wiring density below a certain limit and thus to increase the number of conductors connecting to common electrodes. FIGS. 22 and 23 are each a wiring diagram of part of this configuration.
In FIGS. 22 and 23, every four light-emitting sections 126 have their anodes connected together to one individual electrode 128, and the four light-emitting sections 126 forming each group have their cathodes connected, individually by way of contact holes 125, each to one of conductors 127-1 to 127-4, which serve as common electrodes. One of the four light-emitting sections 126 that connect by way of a contact hole 128 to the output terminal DOa of the first drive section is connected to the output terminal CD1 of the second drive section. This output terminal CD1 is connected by way of the contact hole 129 to the conductor 127-1. The other three of the four light-emitting sections 126 that connect by way of the contact hole 128 to the output terminal DOa of the first drive section are connected each to one of the conductors 127-2 to 127-4, which serve as the common electrodes.
Moreover, one of the four light-emitting sections 126 that connect by way of a contact hole 128 to the output terminal DOb of the first drive section is connected to the output terminal CD2 of the second drive section. This output terminal CD2 is connected by way of the contact hole 129 to the conductor 127-2. The other three of the four light-emitting sections 126 that connect by way of the contact hole 128 to the output terminal DOb of the first drive section are connected each to one of the conductors 127-1, 127-3, and 127-4, which serve as the common electrodes. Furthermore, one of the four light-emitting sections 126 that connect to each of the individual electrodes 128 connected to the output terminals DOc and DOd of the first drive section is connected to one of the output terminals CD3 and CD4 of the second drive section in similar manners. Between FIGS. 22 and 23, the positions at which the contact holes 128 and 125 are formed are reversed. In this way, this configuration requires complicated wiring.
Moreover, the second drive section, which functions as a common electrode selector, generates a large amount of heat, which may cause variation in the output characteristics of the driving IC or otherwise have adverse effects, resulting in large variation in the drive current output from the first drive section. This problem has particularly serious consequences in a case where a large current is fed to a light-emitting device in an optical print head or the like for use in a high-speed, high-resolution printer.
An object of the present invention is to provide an optical print head that operates with smaller differences in brightness among the individual light-emitting sections and with less effects of the heat generated to feed a voltage to the common electrodes on the drive current fed to the individual electrodes.